What is common-mode transient immunity? Why is this required for IC couplers?

Common-mode transient immunity is the maximum tolerable rate of rise (fall) of input/output common-mode voltage at which the specified High (Low) level can be maintained. A photocoupler has a small capacitance (0.6–0.8pF) between the input and output terminals. When an abrupt change in voltage occurs between the input and output terminals, a displacement current that occurs through this capacitance flows into the photodetector chip circuit, eventually causing malfunction. This phenomenon is likely to be felt more clearly in high-speed and high-sensitivity IC-output photocouplers, and thus calls for attention. As a method of preventing it, a shielding layer is provided on the surface of the photodetector chip to let noise signals flow to GND and prevent them from entering the photodetector circuit. This method applies to most of Toshiba IC coupler product groups.